The present invention relates to a device and a method for monitoring a thread wound on a bobbin during operation in a sewing machine.
It is known from various investigations that a bobbin on a sewing machine runs in a very uncontrolled manner during the sewing process. For example, a marking-off of the rotary movement takes place after the beginning of each sewing process, the time of the marking-off and the strength of the incipient rotary movement and the acceleration of the bobbin depending, among other things, on the degree of filling of the bobbin, on the material of the thread and the wear of the mechanical elements in the area of the bobbin, to mention just a few of the possible influential factors. The movement of the bobbin thus cannot substantially be predicted and as a consequence, cannot be described mathematically even when considered from the start of the sewing process.
These general statements apply both to the upper thread and to the lower thread of a sewing machine. However, since any type of monitoring and control in the area of the lower thread poses a major technical challenge because of the very confined spatial proportion, without restricting the invention to this particular case, only monitoring of the lower thread during operation of a sewing machine will be discussed hereinafter.
Various proposed solutions for monitoring a lower thread of a sewing machine for thread breaks or broken threads are known from the prior art. Starting from devices for residual thread monitoring in a lower bobbin, an optical device is known from DE 30 14 753 C2, for example, comprising a light-emitting diode and a photodetector which passes light through holes directed parallel to the axis of rotation of the lower bobbin when the lower thread on the lower bobbin is largely used up. Monitoring of the filling level of the bobbin is therefore performed using the dark/light transition.
In a further development according to DE 34 47 138 C2, a reflecting surface on the bobbin is used to arrange a transmitter and detector substantially as a compact unit only on one side of the lower bobbin on the sewing machine. Incident radiation is strongly absorbed by the thread through holes whilst webs between the holes and a metal rear wall of the bobbin reflect strongly. In this case, a constant continuous signal from the receiver unit is assessed as an indicator that the bobbin is at a standstill which can also occur as a consequence of a thread break.
In order to increase the reliability in this environment exposed to dust and thread abrasion, DE 41 15 520, for example, proposes a special guidance of the lower thread in the course of unwinding the lower bobbin. By this means falsifying impairments of the reflection caused by contaminant accumulations on the reflecting surfaces should be continuously cleaned by quasi-permanent wiping of these surfaces by the running-off thread.
For further monitoring tasks in the area of the lower bobbin, reference is made to the teaching of DE 35 40 126 A1 as an example. In this teaching, in order to achieve more extensive monitoring tasks, three optical sensor cells must cooperate in interplay with two different annular markings or marking units to be applied to a front side of the bobbin.
It is thus the object of the present invention to further develop a method and a device of the type specified initially whilst reducing the expenditure on apparatus and production technology with increased monitoring reliability.
This object is achieved by the features of the independent claims. Advantageous further developments are the subject matter of the dependent claims.